Many modern electrical imaging devices, such as modern electron emission microscopes (EEMs), image an object of interest (also called specimen or sample) by first accelerating and focusing electrons emitted from the object using an electric/magnetic objective lens, and then further magnifying the image by using a series of electric/magnetic projector lenses. An electric/magnetic lens typically generates electric or magnetic fields, or a mix of both, in the path of a beam of emitted electrons for altering the trajectories of the emitted electrons, analogous to the way a glass lens alters the trajectory of a beam of light.
A typical conventional EEM has a radiation source for illuminating an object to be imaged, an electron detector for detecting electrons emitted from the object as a result of the radiation, and one or more electric/magnetic lenses for directing the emitted electrons towards the electron detector to form a magnified image of the object at the detector. In a typical EEM, an electrostatic objective lens is positioned close to the object and has an electrical potential higher than that of the object (usually in the range of 10 to 15 kV) for extracting and accelerating the emitted electrons.
The focal length of such an electric/magnetic lens varies with the kinetic energy of the electrons focused by it. That is, electrons of different energies will be focused at different focal points. For example, the focal length of a magnetic lens for an electron beam of kinetic energy E may be approximated by                                           1            f                    =                                    1                              8                ⁢                m                ⁢                                                                   ⁢                E                                      ⁢                                          ∫                                  z                  1                                                  z                  2                                            ⁢                                                B                  z                  2                                ⁢                                  ⅆ                  z                                                                    ,                            (        1        )            where m is the electron mass, and Bz is the axial magnetic field distribution. As such, a beam of electrons with dispersed energies is not focused in a single plane by an electric/magnetic lens. Not focusing a beam in a single plane results in distortion of the image formed on a single plane. This distortion due to variation in kinetic energy of the emitted electrons is referred to as chromatic aberration. Thus, the spatial resolution of electric/magnetic lenses is limited in part by chromatic aberration.
In EEM, the initial kinetic energies of secondary electrons excited by X-ray radiation typically range from several to tens of electron-volts, and, consequently, chromatic aberration limits spatial resolution of an X-ray photoelectron emission microscope (XPEEM) to about 100 nm.
Several techniques have been developed for reducing chromatic aberration in EEM. The general approach is to filter out emitted electrons having kinetic energies outside a certain range. For example, contrast apertures and Wien filters have been used for such a purpose, improving spatial resolution of XPEEMs to about 20 nm. However, when emitted electrons of varied energies are eliminated, the intensity of emitted electrons arriving at the detector is reduced. Further, certain information is lost. Particularly, a full emission spectrum cannot be obtained.
An alternative approach is to operate an EEM in a time of flight (TOF) mode as, for example, described in “Time-of-Flight Photoelectron Emission Microscopy TOF-PEEM: first results”, Nuclear Instruments And Methods in Physics Research, A 406, (1998), 499-506, H. Spiecker et al, (“Spiecker”). Spiecker discloses a photoelectron emission microscope (PEEM) with a pulsed radiation source. Emitted electrons are dispersed in a drift tube downstream of the imaging optics. The electrons are retarded at the entrance of the drift tube from ˜700 eV to a drift energy of less than 100 eV and are imaged on a multi-channel plate (MCP) at the end of the tube. The electrons are then accelerated to a scintillator screen having a short decay time. Emitted electrons produced from a single radiation pulse are spatially separated in the drift tube due to the spread in their kinetic energy as the drift time of each electron is dependent on its drift energy. The images formed by electrons in different energy ranges are separately recorded in time. The chromatic aberration in each single image is reduced because the energy spread in electrons forming a single image is smaller than the spread in all transmitted electrons. However, this approach has several limitations. Specifically, only electrons within a narrow energy range are collected for each image and they represent only a small fraction of all of the emitted electrons. Accordingly, the acquisition time for a single image is long. As the TOF spectrometer is placed after the imaging optics, initial electron angular motions tend to limit separation of electrons with different energies within the drift tube, and hence the final spatial resolution of the image. Further, it is difficult to integrate this type of TOF spectrometer into a conventional XPEEM.
Another approach is to use a tetrode mirror for correcting both chromatic and spherical aberration, as described in “SMART electron optics”, in 12th European Congress on Electron Microscopy, Proceedings Volume III, Instrumentation and Methodology, (2000), 81-4, D. Preikszas et al. The spatial resolution can be improved to about 2 nm with this approach. However, this approach requires complicated, precise design and positioning of the various lenses and, particularly, the tetrode mirror.
Thus, there is a need for an improved imaging device using emitted electrons with low chromatic aberration wherein both high spatial resolution and full spectrum of emitted electrons can be obtained simultaneously.